Among a series of compounds similar in skeletal structure to the compounds of the present invention, there are maytansine and related compounds (maytanprine, maytanbutine, maytanvaline, maytanbutacine, etc.) isolated from plants by S. M. Kupchan et al.*, as well as colubrinol and colubrinol acetate obtained by M. C. Wani et al.** All of these are potent and characteristic mitosis inhibitors and have antitumor activity. However, since natural resources contain only trace amounts of these compounds, there have been the problem of supply thereof. FNT *S. M. Kupchan et al., J. Amer. Chem. Soc., 94, 1354 (1972); S. M. Kupchan et al., J. Chem. Soc., Chem. Comm., 1972, 1605: S. M. Kupchan et al., J. Org. Chem., 42, 2349 (1977). FNT **M. C. Wani et al., J. Chem. Soc., Chem. Comm., 1973, 390.
Recently, Higashide et al.* have found among the metabolites produced by a microorganism regarded as belonging to the genus Nocardia a group of ansamitocin compounds which have the same skeletal structure as the above-mentioned compounds have but are different therefrom with regard to the ester side chain in position 3, and furthermore they have ascertained that said ansamitocin compounds have excellent antitumor activity at least comparable to that of maytansine. FNT *E. Higashide et al., Nature, vol. 270, 271 (1977).